A fuser in the past includes, near a fixing belt, a heating member for heating the fixing belt and includes, on the inside of the fixing belt, an auxiliary heat generating member that indirectly heats the fixing belt. When a sheet having a size smaller than the width of the fixing belt passes the center of the fixing belt having a predetermined temperature, the temperature of only the center drops. A heating member heats the entire fixing belt in order to raise the temperature of the center where the temperature drops. Therefore, when the temperature of the center reaches the predetermined temperature, the ends of the fixing belt where the sheet does not pass are excessively heated. The temperature of the ends rises to be higher than the temperature of the center. As a result, in the fixing belt, temperature unevenness occurs in the longitudinal direction of the fixing belt.